Display device and manufacturing method thereof

ABSTRACT

A novel display device that is highly convenient or reliable is provided. The display device includes a first display element, a second display element, a first transistor, a second transistor, and a third transistor. The first display element includes a liquid crystal layer. The second display element includes a light-emitting layer. The first transistor has a function of selecting the first display element. The second transistor has a function of selecting the second display element. The third transistor has a function of controlling the driving of the second display element. The first transistor and the second transistor are formed over the same surface. The third transistor is formed above the first transistor and the second transistor and includes one of a source electrode and a drain electrode of the second transistor as a gate electrode.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a display device anda manufacturing method thereof.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. Furthermore, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specific examples of the technical field ofone embodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, a method for drivingany of them, and a method for manufacturing any of them.

2. Description of the Related Art

A liquid crystal display device in which a surface-emitting light sourceis provided as a backlight and combined with a transmissive liquidcrystal display device in order to reduce power consumption and suppressa reduction in display quality is known (see Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-248351

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel display device that is highly convenient or reliable.

Another object of one embodiment of the present invention is to providea display device with low power consumption and high display quality.Another object of one embodiment of the present invention is to providea novel display device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a display device including afirst display element, a second display element, a first transistor, asecond transistor, and a third transistor. The first display elementincludes a liquid crystal layer. The second display element includes alight-emitting layer. The first transistor has a function of selectingthe first display element. The second transistor has a function ofselecting the second display element. The third transistor has afunction of controlling the driving of the second display element. Thefirst transistor and the second transistor are formed over the samesurface. The third transistor is formed above the first transistor andthe second transistor and includes one of a source electrode and a drainelectrode of the second transistor as a gate electrode.

Another embodiment of the present invention is a display deviceincluding a first display element, a second display element, a firsttransistor, a second transistor, a third transistor, and a capacitor.The first display element includes a first pixel electrode and a liquidcrystal layer. The second display element includes a second pixelelectrode and a light-emitting layer. The first transistor iselectrically connected to the first pixel electrode. The secondtransistor is electrically connected to the second pixel electrode. Thethird transistor is electrically connected to the second displayelement. The capacitor includes a pair of electrodes. One of the pair ofelectrodes includes a capacitor electrode. The other of the pair ofelectrodes includes the first pixel electrode. The first transistor andthe second transistor are formed over the same surface. The thirdtransistor is formed above the first transistor and the secondtransistor and includes one of a source electrode and a drain electrodeof the second transistor as a gate electrode.

In the above embodiment, the capacitor electrode is preferably arrangedbelow one or both of the first transistor and the second transistor.

In the above embodiment, the first pixel electrode preferably has afunction of reflecting light, and the second pixel electrode preferablyhas a function of transmitting light.

In the above embodiment, the first pixel electrode includes one or bothof silver and aluminum. The second pixel electrode preferably includesone or more of indium, zinc, tin, and silicon.

In the above embodiment, the third transistor preferably includes aplurality of gate electrodes.

In the above embodiment, one or more of the first transistor, the secondtransistor, and the third transistor preferably include an oxidesemiconductor in a semiconductor layer.

In the above embodiment, the light-emitting layer preferably has afunction of emitting light toward the liquid crystal layer side.

Another embodiment of the present invention is a display moduleincluding a touch sensor and the display device with any one of theabove structures. Another embodiment of the present invention is anelectronic device including a battery and the display device with anyone of the above structures or the display module.

One embodiment of the present invention can provide a novel displaydevice that is highly convenient or reliable. According to oneembodiment of the present invention, a display device with low powerconsumption and high display quality can be provided. According to oneembodiment of the present invention, a novel display device can beprovided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating apixel included in a display device.

FIG. 2 is a cross-sectional view illustrating a pixel included in adisplay device.

FIGS. 3A and 3B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 4A and 4B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 5A and 5B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 6A and 6B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 7A and 7B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 8A and 8B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 9A and 9B are a top view and a cross-sectional view illustrating amethod for manufacturing a pixel included in a display device.

FIGS. 10A and 10B are a top view and a cross-sectional view illustratinga method for manufacturing a pixel included in a display device.

FIGS. 11A and 11B are a top view and a cross-sectional view illustratinga method for manufacturing a pixel included in a display device.

FIGS. 12A and 12B are a top view and a cross-sectional view illustratinga method for manufacturing a pixel included in a display device.

FIGS. 13A and 13B are a top view and a cross-sectional view illustratinga method for manufacturing a pixel included in a display device.

FIG. 14 is a schematic view illustrating a display region of a displayelement.

FIG. 15 is a block diagram illustrating a display device.

FIG. 16 is a circuit diagram illustrating a pixel.

FIGS. 17A and 17B are perspective views illustrating an example of atouch panel.

FIG. 18 is a cross-sectional view illustrating an example of a touchsensor.

FIG. 19 is a cross-sectional view illustrating an example of a touchpanel.

FIGS. 20A and 20B are a block diagram and a timing chart of a touchsensor.

FIGS. 21A to 21C each illustrate an atomic ratio range of an oxidesemiconductor film.

FIGS. 22A to 22C are band diagrams of stacked-layer structures of oxidesemiconductor films.

FIG. 23 illustrates a display module.

FIGS. 24A to 24E each illustrate an electronic device.

FIGS. 25A to 25E are perspective views each illustrating a displaydevice.

FIGS. 26A and 26B show display results of a display device of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be hereinafter described with reference to drawings.Note that the embodiments can be implemented in many different modes. Itwill be readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Therefore, the present inventionshould not be interpreted as being limited to the description in thefollowing embodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Ordinal numbers such as “first,” “second,” and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically.

In this specification, terms for describing arrangement, such as “over”and “under,” are used for convenience for describing the positionalrelation between components with reference to drawings. The positionalrelation between components is changed as appropriate in accordance witha direction in which each component is described. Thus, there is nolimitation on terms used in this specification, and description can bemade appropriately depending on the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. The transistorincludes a channel region between the drain (a drain terminal, a drainregion, or a drain electrode) and the source (a source terminal, asource region, or a source electrode), and current can flow between thesource and the drain through the channel region. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be replaced witheach other in this specification and the like.

In this specification and the like, the term “electrically connected”includes the case where components are connected through an objecthaving any electric function. There is no particular limitation on an“object having any electric function” as long as electric signals can betransmitted and received between components that are connected throughthe object. Examples of an “object having any electric function” includea switching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions, as well as anelectrode and a wiring.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention and the manufacturing method thereof are described withreference to FIG. 1A to FIG. 16.

<1-1. Structure of Display Device>

First, the structure of the display device is described with referenceto FIG. 15. A display device 500 illustrated in FIG. 15 includes a pixelportion 502, and gate driver circuit portions 504 a and 504 b and asource driver circuit portion 506 which are placed outside the pixelportion 502.

[Pixel Portion]

The pixel portion 502 includes pixels 10(1, 1) to 10(X, Y) arranged in Xrows (X is a natural number of 2 or more) and Y columns (Y is a naturalnumber of 2 or more). Each of the pixels 10(1, 1) to 10(X, Y) includestwo display elements having different functions. One of the two displayelements has a function of reflecting incident light, and the other hasa function of emitting light. Note that the details of the two displayelements are described later.

[Gate Driver Circuit Portion]

Some or all of the gate driver circuit portions 504 a and 504 b and thesource driver circuit portion 506 are preferably formed over a substrateover which the pixel portion 502 is formed. Thus, the number ofcomponents and the number of terminals can be reduced. In the case wheresome or all of the gate driver circuit portions 504 a and 504 b and thesource driver circuit portion 506 are not formed over the substrate overwhich the pixel portion 502 is formed, a separately prepared drivercircuit board (e.g., a driver circuit board formed using a singlecrystal semiconductor film or a polycrystalline semiconductor film) maybe formed in the display device 500 by chip on glass (COG) or tapeautomated bonding (TAB).

The gate driver circuit portions 504 a and 504 b have a function ofoutputting a signal (a scan signal) for selecting the pixels 10(1, 1) to10(X, Y). The source driver circuit portion 506 has a function ofsupplying a signal (data signal) for driving the display elementsincluded in the pixels 10(1, 1) to 10(X, Y).

The gate driver circuit portion 504 a has a function of controlling thepotentials of wirings supplied with scan signals (hereinafter, suchwirings are referred to as a scan line GL_L[m], a scan line GL_L[m+1],and a scan line GL_L[X]) or a function of supplying an initializationsignal. The gate driver circuit portion 504 b has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as a scan line GL_E1[m], ascan line GL_E1[m+1], a scan line GL_E2[m], a scan line GL_E2[m+1], ascan line GL_E1[X], and a scan line GL_E2[X]) or a function of supplyingan initialization signal. In the above, m is a natural number less thanor equal to X.

Without being limited to the above functions, the gate driver circuitportions 504 a and 504 b may each have a function of controlling orsupplying another signal.

Although the structure in which the two gate driver circuit portions 504a and 504 b are provided as gate driver circuit portions is illustratedin FIG. 15, the number of gate driver circuit portions is not limitedthereto, and one or three or more gate driver circuit portions may beprovided.

The source driver circuit portion 506 has a function of generating adata signal to be written to the pixels 10(1, 1) to 10(X, Y) which isbased on an image signal, a function of controlling the potentials ofwirings to which data signals are supplied (signal lines SL_L[n],SL_L[n+1], SL_L[Y], SL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n],SL_E2[n+1], and SL_E2[Y]), or a function of supplying an initializationsignal. Note that in the above description, n is a natural number lessthan or equal to Y.

Without being limited to the above functions, the source driver circuitportion 506 may have a function of generating or supplying anothersignal.

The source driver circuit portion 506 includes a plurality of analogswitches or the like. The source driver circuit portion 506 can output,as data signals, time-divided image signals obtained by sequentiallyturning on the plurality of analog switches.

Although the structure where one source driver circuit portion 506 isprovided is illustrated in FIG. 15, the number of the source drivercircuit portions is not limited thereto, and a plurality of sourcedriver circuit portions may be provided in the display device 500. Forexample, two source driver circuit portions may be provided so that thesignal lines SL_L[n], SL_L[n+1], and SL_L[Y] are controlled by one ofthe source driver circuit portions and the signal lines SL_E1[n],SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y] are controlledby the other of the source driver circuit portions.

A pulse signal is input to each of the pixels 10(1, 1) to 10(X, Y)through one of the scan lines GL_L[m], GL_L[m+1], and GL_L[X]. A datasignal is input to each of the pixels 10(1, 1) to 10(X, Y) through oneof the signal lines SL_L[n], SL_L[n+1], SL_L[Y], SL_E1[n], SL_E1[n+1],SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y].

For example, to the pixel 10(m, n) in the m-th row and the n-th column,a pulse signal is input from the gate driver circuit portion 504 athrough the scan line GL_L[m], and a data signal is input from thesource driver circuit portion 506 through the signal line SL_L[n] inaccordance with the potential of the scan line GL_L[m].

For example, the pixel 10(m, n) in the m-th row and the n-th column issupplied with pulse signals from the gate driver circuit portion 504 bthrough the scan lines GL_E1[m] and GL_E2[m] and supplied with a datasignal from the source driver circuit portion 506 through the signallines SL_E1[n] and SL_E2[n] in accordance with the potentials of thescan lines GL_E1[m] and GL_E2[m].

The pixel 10(m, n) includes two display elements as described above. Thescan lines GL_L[m], GL_L[m+1], and GL_L[X] are wirings which control thepotential of one of the two display elements. The scan lines GL_E1[m],GL_E1[m+1], GL_E1[X], GL_E2[m], GL_E2[m+1], and GL_E2[X] are wiringswhich control the potential of the other of the two display elements.

The scan lines SL_L[n], SL_L[n+1], and SL_L[Y] are wirings which controlthe potential of one of the two display elements. The signal linesSL_E1[n], SL_E1[n+1], SL_E1[Y], SL_E2[n], SL_E2[n+1], and SL_E2[Y] arewirings which control the potential of the other of the two displayelements.

[External Circuit]

An external circuit 508 is connected to the display device 500. Notethat the display device 500 may include the external circuit 508.

As shown in FIG. 15, the external circuit 508 is electrically connectedto wirings supplied with anode potentials (hereinafter referred to aswirings ANODE).

<1-2. Circuit Configuration of Pixels>

Next, the circuit configuration of the pixel 10(m, n) is described withreference to FIG. 16.

FIG. 16 is a circuit diagram showing the pixel 10(m, n) which isincluded in the display device 500 of one embodiment of the presentinvention.

The pixel 10(m, n) includes the scan lines GL_L[m], GL_E1[m], andGL_E2[m] and signal lines SL_E1[n], SL_L[n], and SL_E2[n]. The pixel10(m, n) includes transistors MA1 to MA4, a transistor MA5, transistorsMB1 to MB4, a capacitor Cs_L, a display element 12, and a displayelement 14. In FIG. 16, the display element 14 includes display elements14B, 14G, 14R, and 14W. The display element 14B has a function ofemitting blue light. The display element 14G has a function of emittinggreen light. The display element 14R has a function of emitting redlight. The display element 14W has a function of emitting white light.

The pixel 10(m, n) includes a wiring TCOM which is electricallyconnected to the display element 12 and the display element 14, a wiringCSCOM which is electrically connected to the display element 12, and awiring ANODE and a wiring CATHODE which are electrically connected tothe display element 14.

Each of the scan line GL_L[m], the signal line SL_L[n], the wiringCSCOM, and the wiring TCOM is a wiring for driving the display element12. Each of the scan line GL_E1[m], the scan line GL_E2[m], the signalline SL_E1[n], the signal line SL_E2[n], the wiring ANODE, the wiringCATHODE, and the wiring TCOM is a wiring for driving the display element14.

<1-3. Structure Example of First Display Element>

The display element 12 has a function of controlling transmission orreflection of light. In particular, the display element 12 is preferablya reflective display element which controls reflection of light. Thedisplay element 12 serving as a reflective display element can reducepower consumption of the display device because display can be performedwith the use of external light. For example, the display element 12 mayhave a combined structure of a reflective film, a liquid crystalelement, and a polarizing plate, a structure using a micro electromechanical systems (MEMS), or the like.

<1-4. Structure Example of Second Display Element>

The display element 14 has a function of emitting light. Therefore, thedisplay element 14 may be rephrased as a light-emitting element. Forexample, an electroluminescence element (also referred to as an ELelement), or a light-emitting diode may be used as the display element14.

As described above, in the display device of one embodiment of thepresent invention, display elements with different functions like thedisplay elements 12 and 14 are used. In the case where a reflectiveliquid crystal element is used as one of the display elements and atransmissive EL element is used as the other of the display elements, anovel display device that is highly convenient or reliable can beprovided. Furthermore, a display device with low power consumption andhigh display quality can be provided when a reflective liquid crystalelement is used in an environment with bright external light and atransmissive EL element is used in an environment with weak externallight.

<1-5. Driving Method for Display Element>

Next, a driving method of the display element 12 and the display element14 is described with reference to FIG. 16. Note that in the followingdescription, a liquid crystal element is used as the display element 12,and a light-emitting element is used as the display element 14 (thedisplay elements 14B, 14G, 14R, and 14W).

<Driving Method for First Display Element>

In the pixel 10(m, ii), a gate electrode of the transistor MA5 iselectrically connected to the scan line GL_L[m]. One of a sourceelectrode and a drain electrode of the transistor MA5 is electricallyconnected to the signal line SL_L[n], and the other is electricallyconnected to one of a pair of electrodes of the display element 12. Thetransistor MA5 has a function of controlling whether to write a datasignal by switching the on and off states.

The other of the pair of electrodes of the display element 12 iselectrically connected to the wiring TCOM.

One of a pair of electrodes of the capacitor Cs_L is electricallyconnected to the other of the source electrode and the drain electrodeof the transistor MA5 and one of a pair of electrodes of the displayelement 12, and the other of the pair of electrodes of the capacitorCs_L is electrically connected to the wiring CSCOM. The capacitor Cs_Lhas a function of storing data written to the pixel 10(m, n).

For example, the gate driver circuit portion 504 a in FIG. 15 selectsthe pixels 10(m, 1) to 10(m, Y) to turn on the transistors MA5, and dataof data signals are written. When the transistor MA5 is turned off, thepixels 10(m, 1) to (m, Y) to which the data has been written is broughtinto a holding state. This operation is sequentially performed row byrow; thus, an image can be displayed.

[Driving Method for Second Display Element]

In the pixel 10(m, n), a gate electrode of the transistor MA1 iselectrically connected to the scan line GL_E1[m]. One of a sourceelectrode and a drain electrode of the transistor MA1 is electricallyconnected to the signal line SL_E1[n] and the other of the sourceelectrode and the drain electrode is electrically connected to a gateelectrode of the transistor MB1 and the wiring TCOM. The transistor MA1has a function of controlling whether to write a data signal byswitching the on and off states.

One of a source electrode and a drain electrode of the transistor MB1 iselectrically connected to one of a pair of electrodes of the displayelement 14B. The other of the source electrode and the drain electrodeof the transistor MB1 is electrically connected to the wiring ANODE. Theother of the pair of electrodes of the display element 14B iselectrically connected to the wiring CATHODE. The transistor MB1functions as what is called a driving transistor which controls currentsupplied to the display element 14B.

A capacitor is formed between the other of the source electrode and thedrain electrode of the transistor MA1 and the wiring ANODE. Thecapacitor has a function of storing data written to the pixel 10(m, n).The transistor MB1 includes a back gate electrode. The back gateelectrode of the transistor MB1 is electrically connected to one of thesource electrode and the drain electrode thereof.

In the pixel 10(m, n), a gate electrode of the transistor MA2 iselectrically connected to the scan line GL_E1[m]. One of a sourceelectrode and a drain electrode of the transistor MA2 is electricallyconnected to the signal line SL_E2[n] and the other of the sourceelectrode and the drain electrode is electrically connected to a gateelectrode of the transistor MB2 and the wiring TCOM. The transistor MA2has a function of controlling whether to write a data signal byswitching the on and off states.

One of a source electrode and a drain electrode of the transistor MB2 iselectrically connected to one of a pair of electrodes of the displayelement 14G. The other of the source electrode and the drain electrodeof the transistor MB2 is electrically connected to the wiring ANODE. Thetransistor MB2 functions as a so-called driving transistor whichcontrols current supplied to the display element 14G.

A capacitor is formed between the other of the source electrode and thedrain electrode of the transistor MA2 and the wiring ANODE. Thecapacitor has a function of storing data written to the pixel 10(m, n).The transistor MB2 includes a back gate electrode. The back gateelectrode is electrically connected to one of the source electrode andthe drain electrode of the transistor MB2.

In the pixel 10(m, n), a gate electrode of the transistor MA3 iselectrically connected to the scan line GL_E2[m]. One of a sourceelectrode and a drain electrode of the transistor MA3 is electricallyconnected to the signal line SL_E1[n] and the other of the sourceelectrode and the drain electrode is electrically connected to a gateelectrode of the transistor MB3 and the wiring TCOM. The transistor MA3has a function of controlling whether to write a data signal byswitching the on and off states.

One of a source electrode and a drain electrode of the transistor MB3 iselectrically connected to one of a pair of electrodes of the displayelement 14R. The other of the source electrode and the drain electrodeof the transistor MB3 is electrically connected to the wiring ANODE. Thetransistor MB3 functions as a so-called driving transistor whichcontrols current supplied to the display element 14R.

A capacitor is formed between the other of the source electrode and thedrain electrode of the transistor MA3 and the wiring ANODE. Thecapacitor has a function of storing data written to the pixel 10(m, n).The transistor MB3 includes a back gate electrode. The back gateelectrode is electrically connected to one of the source electrode andthe drain electrode of the transistor MB3.

In the pixel 10(m, n), a gate electrode of the transistor MA4 iselectrically connected to the scan line GL_E2[m]. One of a sourceelectrode and a drain electrode of the transistor MA4 is electricallyconnected to the signal line SL_E2 [n] and the other of the sourceelectrode and the drain electrode is electrically connected to a gateelectrode of the transistor MB4 and the wiring TCOM. The transistor MA4has a function of controlling whether to write a data signal byswitching the on and off states.

One of a source electrode and a drain electrode of the transistor MB4 iselectrically connected to one of a pair of electrodes of the displayelement 14W. The other of the source electrode and the drain electrodeof the transistor MB4 is electrically connected to the wiring ANODE. Thetransistor MB4 functions as a so-called driving transistor whichcontrols current supplied to the display element 14W.

A capacitor is formed between the other of the source electrode and thedrain electrode of the transistor MA4 and the wiring ANODE. Thecapacitor has a function of storing data written to the pixel 10(m, n).The transistor MB3 includes a back gate electrode. The back gateelectrode is electrically connected to one of the source electrode andthe drain electrode of the transistor MB4.

For example, the gate driver circuit portion 504 b in FIG. 15 selectsthe pixels 10(m, 1) to 10(m, Y) to turn on the transistors MA1 to MA4,and data of data signals are written. When the transistors MA1 to MA4are turned off, the pixels 10(m, 1) to 10(m, Y) to which the data hasbeen written are brought into a holding state. Furthermore, the amountof current flowing between the source electrode and the drain electrodeof the transistor MB1 is controlled in accordance with the potential ofthe written data signal. The display element 14 emits light with aluminance corresponding to the amount of flowing current. This operationis sequentially performed row by row; thus, an image can be displayed.

In this manner, two display elements can be controlled separately withthe use of different transistors in the display device of one embodimentof the present invention. Accordingly, a display device having highdisplay quality can be provided.

As illustrated in FIG. 16, the driving transistors of the displayelement 14 (the transistors MB1 to MB4) each include a back gateelectrode, i.e., each of the transistors includes a plurality of gateelectrodes; thus, the reliability or driving capability of thetransistor can be improved. For example, as illustrated in FIG. 16, theback gate electrode is connected to one of the source electrode and thedrain electrode, whereby a potential of the transistor on the backchannel side can be fixed. Although not shown in the drawing, when aback gate electrode is connected to a gate electrode (a first gateelectrode or a front gate electrode), the current drive capability ofthe transistor can be increased.

Each of the transistors (the transistors MA1 to MA5 and the transistorsMB1 to MB4) used in the display device of one embodiment of the presentinvention preferably includes an oxide semiconductor film. Thetransistor including an oxide semiconductor film can have relativelyhigh field-effect mobility and thus can operate at high speed. Theoff-state current of the transistor including an oxide semiconductorfilm is extremely low. Therefore, the luminance of the display devicecan be kept even when the refresh rate of the display device is lowered,so that power consumption can be lowered.

A progressive type display, an interlace type display, or the like canbe employed as the display type of the display element 12 and thedisplay element 14. Further, color components controlled in a pixel atthe time of color display are not limited to blue, green, red, or white.For example, one or more colors of yellow, cyan, magenta, and the likemay be added to blue, green, red, or white. Further, the size of adisplay region may be different in each dot of a color component.However, the display device of one embodiment of the present inventionis not limited to a color display device and can be applied to amonochrome display device.

Note that the display device 500 can perform grayscale display using atleast one of the display elements 12 and 14. For example, since thedisplay element 12 is what is called a reflective liquid crystalelement, visibility can be increased under strong external light. In thecase where grayscale display is performed using the display element 12,backlight or the like is not necessarily adjusted as compared to thecase where a transmissive liquid crystal display device is used; thus,power consumption can be reduced.

On the other hand, since the display element 14 is what is called alight-emitting element, visibility can be improved under weak externallight. In the case where grayscale display is performed using thedisplay element 14, image quality such as contrast can be improved ascompared to the case where a transmissive liquid crystal display deviceis used because light emission can be controlled in each pixel withoutusing a backlight or the like.

The display device 500 may perform grayscale display using both thedisplay elements 12 and 14. When grayscale display is performed usingboth the display elements 12 and 14, visibility can be improved ascompared to the case where grayscale display is performed using one ofthe display elements 12 and 14.

<1-6. Display Region of Display Element>

Here, display regions of the display elements 12 and 14 in the pixel10(m, n) are described with reference to FIG. 14.

FIG. 14 is a schematic view illustrating the display regions of thepixel 10(m, n).

The display regions of the pixel 10(m, n) illustrated in FIG. 14 includea display region 12 d which functions as a display region of the displayelement 12, a display region 14Bd which functions as a display region ofthe display element 14B, a display region 14Gd which functions as adisplay region of the display element 14G, a display region 14Rd whichfunctions as a display region of the display element 14R, and a displayregion 14Wd which functions as a display region of the display element14W.

The display region 12 d includes a region which reflects light. Thedisplay region 14Bd includes a region which transmits blue light. Thedisplay region 14Gd includes a region which transmits green light. Thedisplay region 14Rd includes a region which transmits red light. Thedisplay region 14Wd includes a region which transmits white light.

For example, when the display regions of the pixel 10(m, n) are providedas illustrated in FIG. 14, a monochrome image can be displayed on thedisplay region 12 d and a full-color image can be displayed by thedisplay regions 14Gd, 14Rd, and 14Wd.

<1-7. Structure Example of Display Device (Cross-Section)>

Next, an example of a cross-sectional structure of a pixel 10 includedin the display device 500 is described with reference to FIGS. 1A and1B.

FIG. 1A is an example of a top view of the pixel 10. FIG. 1B is across-sectional view corresponding to cross sections taken alongdashed-dotted lines A1-A2, A3-A4, and A5-A6. Note that in the top viewof the pixel 10 illustrated in FIG. 1A, some components are notillustrated in order to avoid complexity of the drawing. In FIG. 1A,display regions adjacent to each other in the pixel (the regionscorresponding to the display regions 14Rd, 14Gd, 14Bd, and 14Wd) areclearly shown. The same applies to the following top views of the pixel10.

In the pixel 10 illustrated in FIGS. 1A and 1B, the display elements 12and 14 and transistors Tr1, Tr2, and Tr3 are provided between substrates80 and 90.

Note that the transistor Tr1 corresponds to the transistor MA5. Thetransistor Tr2 corresponds to any one of the transistors MA1 to MA4. Thetransistor Tr3 corresponds to any one of the transistors MB1 to MB4.

The display element 12 includes a liquid crystal layer 96. The displayelement 14 includes an EL layer 76. The transistor Tr1 has a function ofselecting the display element 12. The transistor Tr2 has a function ofselecting the display element 14. The transistor Tr3 has a function ofcontrolling driving of the display element 14. The transistors Tr1 andTr2 are formed over the same surface. The transistor Tr3 is formed abovethe transistors Tr1 and Tr2 and includes any one of the source electrodeand the drain electrode of the transistor Tr2 as a gate electrode.

Note that the display element 12 includes a conductive film 36functioning as a first pixel electrode. The transistor Tr1 iselectrically connected to the conductive film 36 and has a function ofselecting the display element 12. The transistor Tr3 is electricallyconnected to a conductive film 70 and has a function of selecting thedisplay element 14.

The pixel 10 includes a capacitor 16. The capacitor 16 includes a pairof electrodes. One of the pair of electrodes includes a conductive film42 functioning as a capacitor electrode. The other of the pair ofelectrodes includes the conductive film 36. Note that the conductivefilm 42 is provided below one or both of the transistors Tr1 and Tr2.

The capacitor 16 corresponds to the capacitor Cs_L described above.

When the conductive film 42 functioning as a capacitor electrode ispositioned below one or both of the transistors Tr1 and Tr2, noise dueto data rewriting of the display element 12, i.e., noise due to datarewriting of the liquid crystal element in the pixel can be reduced.

As illustrated in FIGS. 1A and 1B, the transistors Tr1 and Tr2 areformed over the same surface and the transistor Tr3 is formed above thetransistors Tr1 and Tr2, whereby a circuit area can be reduced. Sincethe transistor Tr3 includes one of the source electrode and the drainelectrode included in the transistor Tr2 as a gate electrode, amanufacturing process can be shortened.

As illustrated in FIGS. 1A and 1B, the transistors Tr1, Tr2, and Tr3 arepreferably transistors each having an inverted staggered structure (alsoreferred to as a bottom-gate structure). The transistor having a bottomgate structure can be manufactured in a relatively easy process.However, one embodiment of the present invention is not limited to this,and transistors each having a top gate structure may be used.

The display element 12 has a function of reflecting incident light. Notethat the display element 12 is what is called a liquid crystal elementand includes the liquid crystal layer 96 between a pair of electrodes.One of the pair of electrodes includes the conductive film 36, and theother of the pair of electrodes includes a conductive film 92. Asillustrated in FIGS. 1A and 1B, the display element 12 may includealignment films 94 and 98 in contact with the liquid crystal layer 96.The conductive film 36 functions as a reflective electrode. When lightwhich enters from the outside is reflected by the conductive film 36 asshown by a dashed arrow in FIG. 1B, the light can be reflected to theviewing side.

The display element 14 has a function of emitting light. Note that thedisplay element 14 is what is called a light-emitting element andincludes the EL layer 76 between a pair of electrodes. One of the pairof electrodes includes the conductive film 70, and the other of the pairof electrodes includes a conductive film 78. The conductive film 78functions as a reflective electrode. As shown by a dashed-two dottedarrow in FIG. 1B, light emitted from the EL layer 76 is reflected by theconductive film 78, passes through the conductive film 70, and isextracted to the liquid crystal layer 96 side. Light emitted from thedisplay element 14 is extracted through an opening provided in theconductive film 36 to the substrate 90 side. In FIG. 1B, the opening isshown as a display region 14 d.

The display element 14 may be a light-emitting element in which the ELlayer 76 emitting light of red (R), green (G), blue (B), and the like isformed using a fine metal mask (FMM). Note that the structure of thedisplay element 14 is not limited thereto, and the display element 14may be an element emitting white (W) light and the light from theelement may pass through coloring films to obtain light of R, G, B, andthe like.

An example of the above structure is shown in FIG. 2. FIG. 2 is across-sectional view of a modification example of the pixel 10illustrated in FIG. 1B. As illustrated in FIG. 2, light from the displayelement 14 is extracted to the outside through a coloring film 69. Asillustrated in FIG. 2, the coloring film 69 is preferably provided tocover part of the transistor Tr1. In particular, when a channel regionof the transistor Tr1 is covered with the coloring film 69, the amountof light which enters the channel region can be reduced. Reducing theamount of light which enters the channel region can increase lightresistance of the transistor Tr1.

<1-8. Manufacturing Method of Display Device>

Next, a manufacturing method of the pixel 10 included in the displaydevice 500 illustrated in FIGS. 1A and 1B is described with reference toFIG. 3A to FIG. 13B.

First, a conductive film 31, an insulating film 32, and an insulatingfilm 34 are formed in this order over a substrate 30. Then, a conductivefilm is formed over the insulating film 34 and processed into an islandshape, whereby the conductive film 36 is formed (see FIGS. 3A and 3B).

The conductive film 31 functions as a first separation layer. Theinsulating films 32 and 34 function as a second separation layer. Theconductive film 36 functions as a reflective film. Note that theconductive film 36 may be called a first pixel electrode. The firstpixel electrode preferably contains one or both of silver and aluminum.The first pixel electrode containing silver and aluminum can increaseits reflectance. Note that examples of the first pixel electrodecontaining silver can include an alloy containing silver, palladium, andcopper.

Next, a conductive film is formed over the insulating film 34 and theconductive film 36 and processed into an island shape, whereby aconductive film 38 is formed. Then, an insulating film 40 is formed overthe conductive films 36 and 38 (see FIGS. 4A and 4B).

The conductive film 38 is formed using a light-transmitting conductivematerial and functions as part of one of the pair of electrodes of thedisplay element 12. Note that the conductive film 38 has the followingfunction. The conductive film 38 has a function of controlling thealignment of the liquid crystal layer 96 included in the display element12. For example, in the case where the conductive film 38 is notprovided, the alignment state of the liquid crystal layer 96 overlappingwith the opening of the conductive film 36 (corresponding to the displayregion 14 d shown in FIG. 1B) cannot be controlled in some cases. Incontrast, in the case where the conductive film 38 is provided, thealignment state of the liquid crystal layer 96 can be appropriatelycontrolled by a potential applied to the conductive films 36 and 38.

Next, a conductive film is formed over the insulating film 40 andprocessed into an island shape, whereby the conductive film 42 isformed. Next, an insulating film 44 is formed over the conductive film42. Then, a conductive film is formed over the insulating film 44 andprocessed into an island shape, whereby conductive films 46 a, 46 b, and46 c are formed. Then, an insulating film 48 is formed over theinsulating film 44, the conductive film 46 a, the conductive film 46 b,and the conductive film 46 c. Then, an oxide semiconductor film isformed over the insulating film 48 and processed into an island shape,whereby oxide semiconductor films 50 a and 50 b are formed. Then, anopening 51 is formed in a desired region of the insulating film 40, theinsulating film 44, and the insulating film 48. Note that the opening 51is formed so as to expose part of the conductive film 36 (see FIGS. 5Aand 5B).

The conductive film 42 functions as the other of the pair of electrodesof the capacitor 16. That is, the capacitor 16 includes the conductivefilm 36, the conductive film 38, the insulating film 40, and theconductive film 42. The conductive films 36 and 38 function as one ofthe pair of electrodes of the capacitor 16. The conductive film 42functions as the other of the pair of electrodes of the capacitor 16.The insulating film 40 functions as a dielectric film of the capacitor16.

The conductive film 46 a functions as a gate electrode of the transistorTr1. The conductive film 46 c functions as a gate electrode of thetransistor Tr2.

Next, a conductive film is formed over the insulating film 48, the oxidesemiconductor film 50 a, the oxide semiconductor film 50 b, and theopening 51 and processed into island shapes, whereby conductive films 52a, 52 b, 52 c, 52 d, and 52 e are formed (see FIGS. 6A and 6B).

The conductive film 52 a functions as a source electrode or a drainelectrode of the transistor Tr1. The conductive film 52 b functions as asource electrode or a drain electrode of the transistor Tr1. Theconductive film 52 c functions as a gate electrode of the transistorTr3. The conductive film 52 d functions as a source electrode or a drainelectrode of the transistor Tr2. The conductive film 52 e functions as asource electrode or a drain electrode of the transistor Tr2.

Next, an insulating film 54 is formed over the insulating film 48, theoxide semiconductor films 50 a and 50 b, and the conductive films 52 a,52 b, 52 c, 52 d, and 52 e. Then, an oxide semiconductor film is formedover the insulating film 54 and processed, whereby an island-shapedoxide semiconductor film 56 is formed. After that, a conductive film isformed over the island-shaped oxide semiconductor film 56 and processedinto island shapes, whereby conductive films 58 a and 58 b are formed(see FIGS. 7A and 7B).

The conductive films 58 a and 58 b function as a source electrode and adrain electrode of the transistor Tr3.

Next, insulating films 60 and 62 are formed over the insulating film 54and the conductive films 58 a and 58 b. Then, an opening 63 reaching theconductive film 58 b is formed in a desired region of the insulatingfilms 60 and 62. Then, a conductive film is formed over the insulatingfilm 62 so as to fill the opening 63 and processed into an island shape,whereby the conductive film 64 is formed (see FIGS. 8A and 8B).

The conductive film 64 functions as a back gate electrode of thetransistor Tr3. The conductive film 64 is electrically connected to theconductive film 58 b.

Next, an insulating film 68 having an opening is formed over theinsulating film 62 and the conductive film 64. Next, a conductive filmis formed over the insulating film 68 and processed into an islandshape, whereby the conductive film 70 is formed. Then, an insulatingfilm 72 having an opening is formed over the insulating film 68 and theconductive film 70 (see FIGS. 9A and 9B).

The insulating film 68 functions as a planarization insulating film. Theconductive film 70 functions as one of a pair of electrodes of thedisplay element 14 and a second pixel electrode. Note that theconductive film 70 is electrically connected to the conductive film 58 band the conductive film 64. The insulating film 72 has a function ofseparating part of the display element 14 from the other part betweenadjacent pixels. The conductive film 70 functioning as the second pixelelectrode preferably contains one or more elements selected from indium,zinc, tin, and silicon. For example, as the conductive film 70, aconductive film containing indium, tin, and silicon can be used. In FIG.9A, hatching of the conductive film 70 is seen transparently so thatcomponents under the conductive film 70 are clearly shown.

Next, a structure body 74 is formed over the insulating film 72. Then,the EL layer 76 and the conductive film 78 are formed over theinsulating film 72 and the structure body 74 (see FIGS. 10A and 10B).

The structure body 74 has a function of making a predetermined gapbetween the display element 14 and the substrate 80. The EL layer 76 hasa function of emitting light. The conductive film 78 functions as theother of the pair of electrodes of the display element 14.

Next, a sealing material 82 is applied on the conductive film 78, andthe substrate 30 on which the transistor, the display element, and thelike are formed and the substrate 80 are bonded to each other (see FIGS.11A and 11B).

Then, the substrate 30 and the substrate 80 are separated from eachother. In this embodiment, the substrate 30 and the substrate 80 areseparated from each other at the vicinity of an interface between theconductive film 31 and the insulating film 32 (see FIGS. 12A and 12B).

When the element is separated at the interface between the element andthe conductive film 31, a polar solvent (typically water), a nonpolarsolvent, or the like is preferably added to the interface between theconductive film 31 and the insulating film 32. For example, it ispreferable to use water in separating the element at the interfacebetween the element and the conductive film 31 because damage caused byelectrification in separation can be reduced.

As the conductive film 31, any of the following materials can be used.The conductive film 31 can have a single-layer structure or astacked-layer structure containing an element selected from tungsten,molybdenum, titanium, tantalum, zinc, ruthenium, rhodium, palladium,osmium, iridium, gallium, and silicon; an alloy material containing anyof the elements; or a compound material containing any of the elements.In the case of a layer containing silicon, a crystal structure of thelayer containing silicon may be amorphous, microcrystal, polycrystal, orsingle crystal.

When the conductive film 31 is formed as a stacked-layer structureincluding a layer containing tungsten and a layer containing an oxide oftungsten, the layer containing tungsten may be formed and an insulatinglayer containing an oxide may be formed thereover so that the layercontaining an oxide of tungsten is formed at the interface between thetungsten layer and the insulating layer. Alternatively, the layercontaining an oxide of tungsten may be formed by performing thermaloxidation treatment, oxygen plasma treatment, dinitrogen monoxide (N₂O)plasma treatment, treatment with a highly oxidizing solution such asozone water, or the like on the surface of the layer containingtungsten. Plasma treatment or heat treatment may be performed in anatmosphere of oxygen, nitrogen, or dinitrogen monoxide alone, or a mixedgas of any of these gasses and another gas. Surface condition of theconductive film 31 is changed by the plasma treatment or heat treatment,whereby adhesion between the conductive film 31 and the insulating film32 formed later can be controlled.

Although the structure where the conductive film 31 is provided isdescribed in this embodiment, the present invention is not limitedthereto. For example, a structure where the conductive film 31 is notprovided may be employed. In that case, an organic resin film may beformed in a region in which the conductive film 31 is formed. As theorganic resin film, for example, a polyimide-based resin film, apolyamide-based resin film, an acrylic-based resin film, an epoxy-basedresin film, or a phenol-based resin film can be used. Note that in thecase where a polyimide-based resin film is used, a photosensitive andthermosetting organic resin material is preferably used. When aphotosensitive and thermosetting organic resin material is used, theorganic resin material can have a shape or the like.

In the case where the organic resin film is used instead of theconductive film 31, as a method for separating the element formed overthe substrate 30, laser light (e.g., excimer laser having a wavelengthof 308 nm or UV laser having a wavelength of 355 nm which is the thirdharmonic of a YAG laser) is irradiated from the lower side of thesubstrate 30 to weaken the organic resin film, whereby separation isconducted at an interface between the substrate 30 and the organic resinfilm, inside the organic resin film, or at an interface between theorganic resin film and the insulating film 32.

In the case where irradiation with laser light is performed, a regionhaving strong adhesion and a region having weak adhesion are formedbetween the substrate 30 and the insulating film 32 by adjustment of theirradiation energy density of the laser light, and then, the element maybe separated from the substrate 30. As the laser light, linear laserlight may be used.

Next, the insulating films 32 and 34 formed below the substrate 80 areremoved, and the rear surface of the conductive films 36 and 38 isexposed (see FIGS. 13A and 13B).

The insulating films 32 and 34 can be removed by a dry etching methodand/or a wet etching method.

Next, an alignment film 98 is formed in contact with the conductivefilms 36 and 38. Then, the substrate 90 over which the conductive film92 and the alignment film 94 are formed is prepared, and a space betweena side of the substrate 80 over which the alignment film 98 is formedand a side of the substrate 90 over which the alignment film 94 isformed is filled with the liquid crystal layer 96, whereby the displaydevice 500 including the pixels 10 shown in FIGS. 1A and 1B can bemanufactured.

<1-9. Components of Display Device>

Next, the components of the display device 500 and the manufacturingmethod thereof illustrated in FIG. 1A to FIG. 13B, FIG. 15, and FIG. 16are described below.

[Substrate]

The substrates 30, 80, and 90 can be formed using a material having heatresistance high enough to withstand heat treatment in the manufacturingprocess.

Specifically, non-alkali glass, soda-lime glass, alkali glass, crystalglass, quartz, sapphire, or the like can be used. Alternatively, aninorganic insulating film may be used. Examples of the inorganicinsulating film include a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and an aluminum oxide film.

The non-alkali glass may have a thickness of greater than or equal to0.2 mm and less than or equal to 0.7 mm, for example. The non-alkaliglass may be polished to obtain the above thickness.

As the non-alkali glass, a large-area glass substrate having any of thefollowing sizes can be used: the 6th generation (1500 mm×1850 mm), the7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm),the 9th generation (2400 mm×2800 mm), and the 10th generation (2950mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, as each of the substrates 30, 80, and 90, asingle-crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon or silicon carbide, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used.

For the substrates 30, 80, and 90, an inorganic material such as a metalmay be used. Examples of the inorganic material such as a metal includestainless steel and aluminum.

Alternatively, for the substrates 30, 80, and 90, an organic materialsuch as a resin, a resin film, or plastic may be used. Examples of theresin film include polyester, polyolefin, polyamide (e.g., nylon oraramid), polyimide, polycarbonate, polyurethane, an acrylic resin, anepoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyether sulfone (PES), and a resin having a siloxane bond.

For the substrates 30, 80, and 90, a composite material of an inorganicmaterial and an organic material may be used. Examples of the compositematerial include a resin film to which a metal plate or a thin glassplate is bonded, a resin film into which a fibrous or particulate metalor a fibrous or particulate glass is dispersed, and an inorganicmaterial into which a fibrous or particulate resin is dispersed.

Each of the substrates 30, 80, and 90 can at least support films orlayers formed thereover or thereunder and may be one or more of aninsulating film, a semiconductor film, and a conductive film.

[Conductive Film]

As each of the conductive films 31, 36, 38, 42, 46 a, 46 b, 46 c, 52 a,52 b, 52 c, 52 d, 52 e, 58 a, 58 b, 64, 70, 78, and 92, a metal filmhaving conductivity, a conductive film having a function of reflectingvisible light, or a conductive film having a function of transmittingvisible light may be used.

A material containing a metal element selected from aluminum, gold,platinum, silver, copper, chromium, tantalum, titanium, molybdenum,tungsten, nickel, iron, cobalt, palladium, and manganese can be used forthe metal film having conductivity. Alternatively, an alloy containingany of the above metal elements may be used.

For the metal film having conductivity, specifically a two-layerstructure in which a copper film is stacked over a titanium film, atwo-layer structure in which a copper film is stacked over a titaniumnitride film, a two-layer structure in which a copper film is stackedover a tantalum nitride film, a three-layer structure in which atitanium film, a copper film, and a titanium film are stacked in thisorder, or the like may be used. In particular, a conductive filmcontaining a copper element is preferably used because the resistancecan be reduced. As an example of the conductive film containing a copperelement, an alloy film containing copper and manganese is given. Thealloy film is favorable because it can be processed by a wet etchingmethod.

As the metal film having conductivity, a conductive macromolecule or aconductive polymer may be used.

For the conductive film having a function of reflecting visible light, amaterial containing a metal element selected from gold, silver, copper,and palladium can be used. In particular, a conductive film containing asilver element is preferably used because reflectance of visible lightcan be improved.

For the conductive film having a function of transmitting visible light,a material containing an element selected from indium, tin, zinc,gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide,an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (alsoreferred to as ITSO), an In—Zn oxide, an In—Ga—Zn oxide, or the like canbe used.

As the conductive film having a function of transmitting visible light,a film containing graphene or graphite may be used. The film containinggraphene can be formed in the following manner: a film containinggraphene oxide is formed and is reduced. As a reducing method, a methodwith application of heat, a method using a reducing agent, or the likecan be employed.

The conductive films 31, 36, 38, 42, 46 a, 46 b, 46 c, 52 a, 52 b, 52 c,52 d, 52 e, 58 a, 58 b, 64, 70, 78, and 92 can be formed by electrolessplating. As a material that can be deposited by electroless plating, forexample, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag,and Pd can be used. It is further favorable to use Cu or Ag because theresistance of the conductive film can be reduced.

When the conductive film is formed by electroless plating, a diffusionprevention film may be formed under the conductive film to preventcomponent elements of the conductive film from diffusing outward. A seedfilm that can make the conductive film grow may be formed between thediffusion prevention film and the conductive film. The diffusionprevention film can be formed by sputtering, for example. For thediffusion prevention film, a tantalum nitride film or a titanium nitridefilm can be used, for example. The seed film can be formed byelectroless plating. For the seed film, a material similar to thematerial for the conductive film that can be formed by electrolessplating can be used.

Alternatively, each of the conductive films can be a conductive filmhaving a function of reflecting visible light and a function oftransmitting visible light by combining a conductive film having afunction of reflecting visible light and a conductive film having afunction of transmitting visible light. For example, one of the pair ofelectrodes included in the display element 14 is a conductive filmhaving a function of reflecting visible light and the other of the pairof electrodes is a conductive film having a function of reflectingvisible light and transmitting visible light. The above structure can bea micro optical resonator (microcavity) structure utilizing a resonanteffect of light between a pair of electrodes, so that the intensity oflight having a specific wavelength can be increased.

Note that the other of the pair of electrodes included in the displayelement 14 (e.g., the conductive film 70) can have a stacked-layerstructure including an In—Sn—Si oxide and an alloy containing silver.The alloy containing silver may be a thin film (e.g., the thickness isless than or equal to 50 nm, preferably less than or equal to 30 nm) sothat visible light can be transmitted.

[Insulating Film]

For the insulating films 32, 34, 40, 44, 48, 54, 60, 62, 68, and 72, aninorganic insulating material, an organic insulating material, or aninsulating composite material including an insulating inorganic materialand an insulating organic material can be used.

Examples of the insulating inorganic material include a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, and an aluminum oxide film. Alternatively, aplurality of the above inorganic materials may be stacked.

Examples of the above organic insulating material include materials thatinclude polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, polyurethane, an acrylic-based resin, anepoxy-based resin, and a resin having a siloxane bond. As the insulatingorganic material, a photosensitive material may be used.

[Oxide Semiconductor Film]

The oxide semiconductor films 50 a, 50 b, and 56 are formed using ametal oxide such as an In-M-Zn oxide (M is Al, Ga, Y, or Sn).Alternatively, an In—Ga oxide or an In—Zn oxide may be used for theoxide semiconductor films 50 a, 50 b, and 56. Note that an oxidesemiconductor film that can be used as the oxide semiconductor films 50a, 50 b, and 56 will be described in detail in Embodiment 3.

[Liquid Crystal Layer]

As examples of the liquid crystal layer 96, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal, ferroelectric liquid crystal, andanti-ferroelectric liquid crystal are given. Alternatively, a liquidcrystal material which exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the like maybe used. Furthermore, a liquid crystal material exhibiting a blue phasemay be used.

For a driving method of the liquid crystal layer 96, an in-planeswitching (IPS) mode, a twisted nematic (TN) mode, an FFS mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the likecan be used. In addition, the liquid crystal layer 96 can be driven by,for example, a vertical alignment (VA) mode such as a multi-domainvertical alignment (MVA) mode, a patterned vertical alignment (PVA)mode, an electrically controlled birefringence (ECB) mode, a continuouspinwheel alignment (CPA) mode, or an advanced super view (ASV) mode canbe used.

[EL Layer]

The EL layer 76 includes at least a light-emitting material. Examples ofthe light-emitting material include an organic compound and an inorganiccompound such as a quantum dot.

The organic compound and the inorganic compound can be formed by anevaporation method (including a vacuum evaporation method), an ink-jetmethod, a coating method, or gravure printing, for example.

Examples of materials that can be used for the organic compound includea fluorescent material and a phosphorescent material. A fluorescentmaterial is preferably used in terms of the lifetime, while aphosphorescent material is preferably used in terms of the efficiency.Furthermore, both of a fluorescent material and a phosphorescentmaterial may be used.

A quantum dot is a semiconductor nanocrystal with a size of severalnanometers and contains approximately 1×10³ to 1×10⁶ atoms. Since energyshift of quantum dots depend on their size, quantum dots made of thesame substance emit light with different wavelengths depending on theirsize; thus, emission wavelengths can be easily adjusted by changing thesize of quantum dots.

Since a quantum dot has an emission spectrum with a narrow peak,emission with high color purity can be obtained. In addition, a quantumdot is said to have a theoretical internal quantum efficiency ofapproximately 100%, which far exceeds that of a fluorescent organiccompound, i.e., 25%, and is comparable to that of a phosphorescentorganic compound. Therefore, a quantum dot can be used as alight-emitting material to obtain a light-emitting element having highemission efficiency. Furthermore, since a quantum dot which is aninorganic compound has high inherent stability, a light-emitting elementwhich is favorable also in terms of lifetime can be obtained.

Examples of a material of a quantum dot include a Group 14 element inthe periodic table, a Group 15 element in the periodic table, a Group 16element in the periodic table, a compound of a plurality of Group 14elements in the periodic table, a compound of an element belonging toany of Groups 4 to 14 in the periodic table and a Group 16 element inthe periodic table, a compound of a Group 2 element in the periodictable and a Group 16 element in the periodic table, a compound of aGroup 13 element in the periodic table and a Group 15 element in theperiodic table, a compound of a Group 13 element in the periodic tableand a Group 17 element in the periodic table, a compound of a Group 14element in the periodic table and a Group 15 element in the periodictable, a compound of a Group 11 element in the periodic table and aGroup 17 element in the periodic table, iron oxides, titanium oxides,spinel chalcogenides, and semiconductor clusters.

Specific examples include, but are not limited to, cadmium selenide;cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zincsulfide; zinc telluride; mercury sulfide; mercury selenide; mercurytelluride; indium arsenide; indium phosphide; gallium arsenide; galliumphosphide; indium nitride; gallium nitride; indium antimonide; galliumantimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide;lead selenide; lead telluride; lead sulfide; indium selenide; indiumtelluride; indium sulfide; gallium selenide; arsenic sulfide; arsenicselenide; arsenic telluride; antimony sulfide; antimony selenide;antimony telluride; bismuth sulfide; bismuth selenide; bismuthtelluride; silicon; silicon carbide; germanium; tin; selenium;tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide;boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide;barium selenide; barium telluride; calcium sulfide; calcium selenide;calcium telluride; beryllium sulfide; beryllium selenide; berylliumtelluride; magnesium sulfide; magnesium selenide; germanium sulfide;germanium selenide; germanium telluride; tin sulfide; tin selenide; tintelluride; lead oxide; copper fluoride; copper chloride; copper bromide;copper iodide; copper oxide; copper selenide; nickel oxide; cobaltoxide; cobalt sulfide; triiron tetraoxide; iron sulfide; manganeseoxide; molybdenum sulfide; vanadium oxide; tungsten oxide; tantalumoxide; titanium oxide; zirconium oxide; silicon nitride; germaniumnitride; aluminum oxide; barium titanate; a compound of selenium, zinc,and cadmium; a compound of indium, arsenic, and phosphorus; a compoundof cadmium, selenium, and sulfur; a compound of cadmium, selenium, andtellurium; a compound of indium, gallium, and arsenic; a compound ofindium, gallium, and selenium; a compound of indium, selenium, andsulfur; a compound of copper, indium, and sulfur; and combinationsthereof. What is called an alloyed quantum dot, whose composition isrepresented by a given ratio, may be used. For example, an alloyedquantum dot of cadmium, selenium, and sulfur is an effective material toobtain blue light because the emission wavelength can be changed bychanging the percentages of the elements.

As the quantum dot, any of a core-type quantum dot, a core-shell quantumdot, a core-multishell quantum dot, and the like can be used. Note thatwhen a core is covered with a shell formed of another inorganic materialhaving a wider band gap, the influence of defects and dangling bondsexisting at the surface of a nanocrystal can be reduced. Since such astructure can significantly improve the quantum efficiency of lightemission, it is preferable to use a core-shell or core-multishellquantum dot. Examples of the material of a shell include zinc sulfideand zinc oxide.

Quantum dots have a high proportion of surface atoms and thus have highreactivity and easily cohere together. For this reason, it is preferablethat a protective agent be attached to, or a protective group beprovided at the surfaces of quantum dots. The attachment of theprotective agent or the provision of the protective group can preventcohesion and increase solubility in a solvent. It can also reducereactivity and improve electrical stability. Examples of the protectiveagent (or the protective group) include polyoxyethylene alkyl etherssuch as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, andpolyoxyethylene oleyl ether; trialkylphosphines such astripropylphosphine, tributylphosphine, trihexylphosphine, andtrioctylphosphine; polyoxyethylene alkylphenyl ethers such aspolyoxyethylene n-octylphenyl ether and polyoxyethylene n-nonylphenylether; tertiary amines such as tri(n-hexyl)amine, tri(n-octyl)amine, andtri(n-decyl)amine; organophosphorus compounds such as tripropylphosphineoxide, tributylphosphine oxide, trihexylphosphine oxide,trioctylphosphine oxide, and tridecylphosphine oxide; polyethyleneglycol diesters such as polyethylene glycol dilaurate and polyethyleneglycol distearate; organic nitrogen compounds such asnitrogen-containing aromatic compounds, e.g., pyridines, lutidines,collidines, and quinolines; aminoalkanes such as hexylamine, octylamine,decylamine, dodecylamine, tetradecylamine, hexadecylamine, andoctadecylamine; dialkylsulfides such as dibutylsulfide;dialkylsulfoxides such as dimethylsulfoxide and dibutylsulfoxide;organic sulfur compounds such as sulfur-containing aromatic compounds,e.g., thiophenes; higher fatty acids such as a palmitin acid, a stearicacid, and an oleic acid; alcohols; sorbitan fatty acid esters; fattyacid modified polyesters; tertiary amine modified polyurethanes; andpolyethyleneimines.

Since band gaps of quantum dots are increased as their size isdecreased, the size is adjusted as appropriate so that light with adesired wavelength can be obtained. Light emission from the quantum dotsis shifted to a blue color side, i.e., a high energy side, as thecrystal size is decreased; thus, emission wavelengths of the quantumdots can be adjusted over a wavelength region of a spectrum of anultraviolet region, a visible light region, and an infrared region bychanging the size of quantum dots. The range of size (diameter) ofquantum dots which is usually used is 0.5 nm to 20 nm, preferably 1 nmto 10 nm. The emission spectra are narrowed as the size distribution ofthe quantum dots gets smaller, and thus light can be obtained with highcolor purity. The shape of the quantum dots is not particularly limitedand may be spherical shape, a rod shape, a circular shape, or the like:Quantum rods which are rod-like shape quantum dots emit directionallight polarized in the c-axis direction; thus, quantum rods can be usedas a light-emitting material to obtain a light-emitting element withhigher external quantum efficiency.

In most EL elements, to improve emission efficiency, light-emittingmaterials are dispersed in host materials and the host materials need tobe substances each having a singlet excitation energy or a tripletexcitation energy higher than or equal to that of the light-emittingmaterial. In the case of using a blue phosphorescent material, it isparticularly difficult to develop a host material which has a tripletexcitation energy higher than or equal to that of the bluephosphorescent material and which is excellent in terms of a lifetime.On the other hand, even when a light-emitting layer is composed ofquantum dots and made without a host material, the quantum dots enableemission efficiency to be ensured; thus, a light-emitting element whichis favorable in terms of a lifetime can be obtained. In the case wherethe light-emitting layer is composed of quantum dots, the quantum dotspreferably have core-shell structures (including core-multishellstructures).

[Alignment Film]

For the alignment films 94 and 98, a material containing polyimide orthe like can be used. For example, a material containing polyimide orthe like may be subjected to a rubbing process or an optical alignmentprocess to have alignment in a predetermined direction.

[Coloring Film]

The color film 69 functions as a color filter. For the color film 69, amaterial transmitting light of a predetermined color (e.g., a materialtransmitting light of blue, green, red, yellow, or white) is used.

[Structure Body]

For the structure body 74, an organic material, an inorganic material,or an insulating material containing a composite material of an organicmaterial and an inorganic material can be used. For the insulatingmaterial, the materials for the insulating films 32, 34, 40, 44, 48, 54,60, 62, 68, and 72 can be used.

[Sealing Material]

For the sealing material 82, an inorganic material, an organic material,a composite material of an inorganic material and an organic material,or the like can be used. Examples of the organic material include athermally fusible resin and a curable resin. As the sealing material 82,an adhesive including a resin material (e.g., a reactive curableadhesive, a photocurable adhesive, a thermosetting adhesive, or ananaerobic adhesive) may be used. Examples of such resin materialsinclude an epoxy-based resin, an acrylic-based resin, a silicone-basedresin, a phenol-based resin, a polyimide-based resin, an imide-basedresin, a polyvinyl chloride (PVC) based resin, a polyvinyl butyral (PVB)based resin, and an ethylene vinyl acetate (EVA) based resin.

Although not illustrated in FIG. 1A to FIG. 13B, FIG. 15, and FIG. 16,the display device 500 may include components described below.

[Functional Film]

The display device 500 may include a functional film in contact with oneor both of the substrates 80 and 90. As the functional film, apolarizing plate, a retardation plate, a diffusing film, ananti-reflective film, a condensing film, or the like can be used.Alternatively, an antistatic film preventing the attachment of a foreignsubstance, a water repellent film suppressing the attachment of stain, ahard coat film suppressing generation of a scratch in use, or the likecan be used for the functional film.

[Light-Blocking Film]

The display device 500 may include a light-blocking film suppressinglight transmission between adjacent pixels. Examples of the material forthe light-blocking film include a metal material and an organic resinmaterial containing a black pigment.

In this manner, two display elements can be controlled separately withthe use of different transistors in the display device of one embodimentof the present invention. Accordingly, a display device having highdisplay quality can be provided.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, the display device of one embodiment of the presentinvention which includes an input device will be described withreference to FIGS. 17A and 17B, FIG. 18, FIG. 19, and FIGS. 20A and 20B.

<2-1. Description of Input Device>

In this embodiment, a touch panel 2000 including a display device 500and an input device will be described. In addition, an example in whicha touch sensor is used as an input device will be described.

FIGS. 17A and 17B are perspective views of the touch panel 2000. Notethat FIGS. 17A and 17B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 500 and a touch sensor2595 (see FIG. 17B). The touch panel 2000 includes a substrate 80, asubstrate 90, and a substrate 2590.

The display device 500 includes a plurality of pixels over the substrate80 and a plurality of wirings 2511 through which signals are supplied tothe pixels. The plurality of wirings 2511 are led to a peripheralportion of the substrate 80, and parts of the plurality of wirings 2511form a terminal 2519. The terminal 2519 is electrically connected to anFPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 17B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 80) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used, forexample. Examples of the capacitive touch sensor include a surfacecapacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self-capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. Note that the touch sensor 2595 illustrated inFIG. 17B is an example of using a projected capacitive touch sensor.Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 17A and 17B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

Note that as a material of the conductive films such as the electrodes2591, the electrodes 2592, and the wirings 2598, that is, wirings andelectrodes forming the touch panel, a transparent conductive filmcontaining indium oxide, tin oxide, zinc oxide, or the like (e.g., ITO)can be given. For example, a low-resistance material is preferably usedas a material that can be used as the wirings and electrodes forming thetouch panel. For example, silver, copper, aluminum, a carbon nanotube,graphene, or a metal halide (such as a silver halide) may be used.Alternatively, a metal nanowire including a plurality of conductors withan extremely small width (for example, a diameter of several nanometers)may be used. Further alternatively, a net-like metal mesh with aconductor may be used. For example, an Ag nanowire, a Cu nanowire, an Alnanowire, an Ag mesh, a Cu mesh, or an Al mesh may be used. For example,in the case of using an Ag nanowire as the wirings and electrodesforming the touch panel, a visible light transmittance of 89% or moreand a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or less can beachieved. Since the above-described metal nanowire, metal mesh, carbonnanotube, graphene, and the like, which are examples of the materialthat can be used as the wirings and electrodes forming the touch panel,have high visible light transmittances, they may be used as electrodesof display elements (e.g., a pixel electrode or a common electrode).

<2-2. Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 18. FIG. 18 corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 17B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film containing graphene may be used aswell. The film containing graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various patterning techniques such asphotolithography.

Examples of a material for the insulating layer 2593 include a resinsuch as an acrylic resin or an epoxy resin, a resin having a siloxanebond, and an inorganic insulating material such as silicon oxide,silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the fore of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<2-3. Description of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 19. FIG. 19 corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 17A.

In the touch panel 2000 illustrated in FIG. 19, the display device 500including the pixels 10 described with reference to FIGS. 1A and 1B andthe touch sensor 2595 described with reference to FIG. 18 are attachedto each other.

The touch panel 2000 illustrated in FIG. 19 includes an adhesive layer2597 and an anti-reflective layer 2569 in addition to the display device500 including the pixels 10 described with reference to FIGS. 1A and 1Band the touch sensor 2595 described with reference to FIG. 18.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 90 so that the touch sensor 2595 overlaps with the displaydevice 500. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2569 is positioned in a region overlappingwith pixels 10. As the anti-reflective layer 2569, a circularlypolarizing plate can be used, for example.

The touch panel 2000 is what is called an out-cell touch panel. Notethat one embodiment of the present invention is not limited to the abovestructure, and the touch panel 2000 may be an in-cell touch panel or anon-cell touch panel.

<2-4. Description of Driving Method of Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 20A and 20B.

FIG. 20A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 20A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 20A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 20A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 20B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 20A. In FIG. 20B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 20B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current value isdecreased at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be detected.

The structures described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 3

In this embodiment, an oxide semiconductor film of one embodiment of thepresent invention is described with reference to FIGS. 21A to 21C andFIGS. 22A to 22C.

<3-1. Oxide Semiconductor Film>

An oxide semiconductor film according to one embodiment of the presentinvention is described below.

An oxide semiconductor film preferably contains at least indium or zinc.In particular, indium and zinc are preferably contained. In addition,aluminum, gallium, yttrium, tin, or the like is preferably contained.Furthermore, one or more elements selected from boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, or the likemay be contained.

Here, the case where an oxide semiconductor film is InMZnO containingindium, an element M, and zinc is considered. The element M is aluminum,gallium, yttrium, tin, or the like. Other elements that can be used asthe element M include boron, silicon, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, and magnesium. Note that two or more of the above elements maybe used in combination as the element M.

<3-2. Structure of Oxide Semiconductor Film>

An oxide semiconductor film is classified into a single crystal oxidesemiconductor film and a non-single-crystal oxide semiconductor film.Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in thea-b plane direction, and its crystal structure has distortion. Note thatthe distortion is a portion where the direction of a lattice arrangementchanges between a region with a regular lattice arrangement and anotherregion with a regular lattice arrangement in a region in whichnanocrystals are connected.

The shape of the nanocrystal is basically a hexagon but is not always aregular hexagon and is a non-regular hexagon in many cases. A pentagonallattice arrangement, a heptagonal lattice arrangement, and the like isincluded in the distortion in some cases. Note that a clear crystalgrain boundary cannot be observed even in the vicinity of distortion inthe CAAC-OS. That is, a lattice arrangement is distorted so thatformation of a crystal grain boundary is inhibited. This is probablybecause the CAAC-OS can tolerate distortion owing to a low density ofarrangement of oxygen atoms in an a-b plane direction, a change ininteratomic bond distance by substitution of a metal element, and thelike.

The CAAC-OS tends to have a layered crystal structure (also referred toas a layered structure) in which a layer containing indium and oxygen(hereinafter, In layer) and a layer containing the element Al, zinc, andoxygen (hereinafter, (M,Zn) layer) are stacked. Note that indium and theelement M can be replaced with each other, and when the element M of the(M,Zn) layer is replaced with indium, the layer can also be referred toas an (In,M,Zn) layer. When indium of the In layer is replaced with theelement M, the layer can also be referred to as an (In,M) layer.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different nanocrystals in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS cannot be distinguished from ana-like OS or an amorphous oxide semiconductor, depending on an analysismethod.

The a-like OS has a structure intermediate between those of the nc-OSand the amorphous oxide semiconductor. The a-like OS has a void or alow-density region. That is, the a-like OS has low crystallinity ascompared with the nc-OS and the CAAC-OS.

An oxide semiconductor can have various structures which show variousdifferent properties. Two or more of the amorphous oxide semiconductor,the polycrystalline oxide semiconductor, the a-like OS, the nc-OS, andthe CAAC-OS may be included in an oxide semiconductor of one embodimentof the present invention.

<3-3. Atomic Ratio of Oxide Semiconductor Film>

First, preferred ranges of the atomic ratio of indium, the element M,and zinc contained in an oxide semiconductor film according to anembodiment of the present invention are described with reference toFIGS. 21A to 21C. Note that the proportion of oxygen atoms is not shownin FIGS. 21A to 21C. The terms of the atomic ratio of indium, theelement M, and zinc contained in the oxide semiconductor film aredenoted by [In], [M], and [Zn], respectively.

In FIGS. 21A to 21C, broken lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):1 (where −1≤α≤1), a line where the atomicratio [In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):5.

In addition, dash-dotted lines indicate a line where the atomic ratio[In]:[M]:[Zn] is 5:1:β (β≥0), a line where the atomic ratio[In]:[M]:[Zn] is 2:1:β, a line where the atomic ratio [In]:[M]:[Zn] is1:1:β, a line where the atomic ratio [In]:[M]:[Zn] is 1:2:β, a linewhere the atomic ratio [In]:[M]:[Zn] is 1:3:β, and a line where theatomic ratio [In]:[M]:[Zn] is 1:4:β.

Dashed-double dotted lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+γ): 2:(1−γ), where −1≤γ≤1. Furthermore, an oxidesemiconductor film with the atomic ratio of [In]:[M]:[Zn]=0:2:1 or aneighborhood thereof in FIGS. 21A to 21C tends to have a spinel crystalstructure.

A plurality of phases (e.g., two phases or three phases) exists in theoxide semiconductor film in some cases. For example, with an atomicratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinelcrystal structure and a layered crystal structure are likely to exist.In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0,two phases of a bixbyite crystal structure and a layered crystalstructure are likely to exist. In the case where a plurality of phasesexist in the oxide semiconductor film, a grain boundary might be Ruinedbetween different crystal structures.

A region A in FIG. 21A represents examples of the preferred ranges ofthe atomic ratio of indium, the element M, and zinc contained in anoxide semiconductor film.

In addition, the oxide semiconductor film containing indium in a higherproportion can have high carrier mobility (electron mobility). This isbecause in an oxide semiconductor film containing indium, the element M,and zinc, the s orbital of heavy metal mainly contributes to carriertransfer, and a higher indium content in the oxide semiconductorenlarges a region where the s orbitals of indium atoms overlap. Thus, anoxide semiconductor film having a high content of indium has highercarrier mobility than an oxide semiconductor film having a low contentof indium.

In contrast, when the indium content and the zinc content in an oxidesemiconductor film become lower, carrier mobility becomes lower. Thus,with an atomic ratio of [In]:[M]:[Zn]=0:1:0 and the neighborhood thereof(e.g., a region C in FIG. 21C), insulation performance becomes better.

Accordingly, an oxide semiconductor film in one embodiment of thepresent invention preferably has an atomic ratio represented by a regionA in FIG. 21A. With this atomic ratio, a layered structure with highcarrier mobility and a few grain boundaries is easily obtained.

An oxide semiconductor film with an atomic ratio in the region A,particularly in a region B in FIG. 21B, is excellent because the oxidesemiconductor film easily becomes a CAAC-OS and has high carriermobility.

The CAAC-OS is an oxide semiconductor with high crystallinity. Incontrast, in the CAAC-OS, a reduction in electron mobility due to thegrain boundary is less likely to occur because a clear grain boundarycannot be observed. Entry of impurities, formation of defects, or thelike might decrease the crystallinity of an oxide semiconductor. Thismeans that the CAAC-OS has small amounts of impurities and defects(e.g., oxygen vacancies). Thus, an oxide semiconductor including aCAAC-OS is physically stable. Therefore, the oxide semiconductorincluding a CAAC-OS is resistant to heat and has high reliability.

Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=4:2:3to 4.1 and the vicinity thereof. The vicinity includes an atomic ratioof [In]:[M]:[Zn]=5:3:4. Note that the region B includes an atomic ratioof [In]:[M]:[Zn]=5:1:6 and the vicinity thereof and an atomic ratio of[In]:[M]:[Zn]=5:1:7 and the vicinity thereof.

Note that the property of an oxide semiconductor film is not uniquelydetermined by an atomic ratio. Even with the same atomic ratio, theproperty of an oxide semiconductor film might be different depending ona formation condition. For example, in the case where the oxidesemiconductor film is formed with a sputtering apparatus, a film havingan atomic ratio deviated from the atomic ratio of a target is formed. Inparticular, [Zn] in the film might be smaller than [Zn] in the targetdepending on the substrate temperature in deposition. Thus, theillustrated regions each represent an atomic ratio with which an oxidesemiconductor film tends to have specific characteristics, andboundaries of the regions A to C are not clear.

<3-4. Transistor Including Oxide Semiconductor Film>

Next, the case where the above-described oxide semiconductor film isused for a transistor will be described.

Note that when the oxide semiconductor film is used for a transistor,carrier scattering or the like at a grain boundary can be reduced; thus,the transistor can have high field-effect mobility. In addition, thetransistor can have high reliability.

An oxide semiconductor film with low carrier density is preferably usedfor the transistor. In order to reduce the carrier density of the oxidesemiconductor film, the impurity concentration in the oxidesemiconductor film is reduced so that the density of defect states canbe reduced. In this specification and the like, a state with a lowimpurity concentration and a low density of defect states is referred toas a highly purified intrinsic or substantially highly purifiedintrinsic state. For example, an oxide semiconductor film whose carrierdensity is lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, orfurther preferably lower than 1×10¹⁰/cm³, and greater than or equal to1×10⁻⁹/cm³ is used as the oxide semiconductor film.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states andaccordingly has few carrier traps in some cases.

Charges trapped by the trap states in the oxide semiconductor film takea long time to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm having a high density of trap states has unstable electricalcharacteristics in some cases.

To obtain stable electrical characteristics of the transistor, it iseffective to reduce the concentration of impurities in the oxidesemiconductor film. In order to reduce the concentration of impuritiesin the oxide semiconductor film, the concentration of impurities in afilm that is adjacent to the oxide semiconductor film is preferablyreduced. As examples of the impurities, hydrogen, nitrogen, alkalimetal, alkaline earth metal, iron, nickel, silicon, and the like aregiven.

<3-5. Impurities in Oxide Semiconductor Film>

Here, the influence of impurities in the oxide semiconductor film willbe described.

When silicon or carbon that is one of Group 14 elements is contained inthe oxide semiconductor film, defect states are formed in the oxidesemiconductor film. Thus, the concentration of silicon or carbon(measured by secondary ion mass spectrometry (SIMS)) in the oxidesemiconductor film and around an interface with the oxide semiconductorfilm is set lower than or equal to 2×10¹⁸ atoms/cm³, and preferablylower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor film contains alkali metal or alkalineearth metal, defect states are formed and carriers are generated, insome cases. Thus, a transistor including an oxide semiconductor filmwhich contains alkali metal or alkaline earth metal is likely to benormally on. Therefore, it is preferable to reduce the concentration ofalkali metal or alkaline earth metal in the oxide semiconductor film.Specifically, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor film, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³.

When the oxide semiconductor film contains nitrogen, the oxidesemiconductor film easily becomes n-type by generation of electronsserving as carriers and an increase of carrier density. Thus, atransistor whose semiconductor includes an oxide semiconductor film thatcontains nitrogen is likely to be normally-on. For this reason, nitrogenin the oxide semiconductor film is preferably reduced as much aspossible; for example, the concentration of nitrogen in the oxidesemiconductor film measured by SIMS is set to be lower than 5×10¹⁹atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁸ atoms/cm³, and still furtherpreferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and thus causes an oxygen vacancy,in some cases. Entry of hydrogen into the oxygen vacancy generates anelectron serving as a carrier in some cases. Furthermore, in some cases,bonding of part of hydrogen to oxygen bonded to a metal atom causesgeneration of an electron serving as a carrier. Thus, a transistorincluding an oxide semiconductor film that contains hydrogen is likelyto be normally on. Accordingly, it is preferable that hydrogen in theoxide semiconductor film be reduced as much as possible. Specifically,the hydrogen concentration in the oxide semiconductor film measured bySIMS is set lower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, and stillfurther preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide semiconductor film with sufficiently reduced impurityconcentration is used for a channel region in a transistor, thetransistor can have stable electrical characteristics.

<3-6. Band Diagram>

Next, the case where the oxide semiconductor film described above has atwo-layer structure or three-layer structure is described.

A band diagram of a stacked-layer structure of an oxide semiconductorfilm S1, an oxide semiconductor film S2, and an oxide semiconductor filmS3 and insulating films that are in contact with the stacked-layerstructure, a band diagram of a stacked-layer structure of the oxidesemiconductor films S2 and S3 and insulating films that are in contactwith the stacked-layer structure, and a band diagram of a stacked-layerstructure of the oxide semiconductor films S1 and S2 and insulatingfilms that are in contact with the stacked-layer structure are describedwith reference to FIGS. 22A to 22C.

FIG. 22A is an example of a band diagram of a layered structureincluding an insulating film I1, the oxide semiconductor film S1, theoxide semiconductor film S2, the oxide semiconductor film S3, and aninsulating film I2 in a thickness direction. FIG. 22B is an example of aband diagram of a layered structure including the insulating film I1,the oxide semiconductor film S2, the oxide semiconductor film S3, andthe insulating film 12 in a thickness direction. FIG. 22C is an exampleof a band diagram of a layered structure including the insulating filmI1, the oxide semiconductor film S1, the oxide semiconductor film S2,and the insulating film 12 in a thickness direction. Note that for easyunderstanding, the band diagrams show the conduction band minimum (Ec)of each of the insulating film I1, the oxide semiconductor film S1, theoxide semiconductor film S2, the oxide semiconductor film S3, and theinsulating film I2.

The conduction band minimum of each of the oxide semiconductor films S1and S3 is closer to the vacuum level than that of the oxidesemiconductor film S2.

Typically, a difference between the conduction band minimum of the oxidesemiconductor film S2 and the conduction band minimum of each of theoxide semiconductor films S1 and S3 is preferably greater than or equalto 0.15 eV or greater than or equal to 0.5 eV, and less than or equal to2 eV or less than or equal to 1 eV. That is, it is preferable that thedifference between the electron affinity of each of the oxidesemiconductor films S1 and S3 and the electron affinity of the oxidesemiconductor film S2 be greater than or equal to 0.15 eV or greaterthan or equal to 0.5 eV, and less than or equal to 2 eV or less than orequal to 1 eV.

As illustrated in FIGS. 22A to 22C, the conduction band minimum of eachof the oxide semiconductor films S1 to S3 is gradually varied. In otherwords, the energy level of the conduction band minimum is continuouslyvaried or continuously connected. In order to obtain such a banddiagram, the density of defect states in a mixed layer formed at aninterface between the oxide semiconductor films S1 and S2 or aninterface between the oxide semiconductor films S2 and S3 is preferablymade low.

Specifically, when the oxide semiconductor films S1 and S2 or the oxidesemiconductor films S2 and S3 contain the same element (as a maincomponent) in addition to oxygen, a mixed layer with a low density ofdefect states can be formed. For example, in the case where the oxidesemiconductor film S2 is an In—Ga—Zn oxide semiconductor film, it ispreferable to use an In—Ga—Zn oxide semiconductor film, a Ga—Zn oxidesemiconductor film, gallium oxide, or the like as each of the oxidesemiconductor films S1 and S3.

At this time, the oxide semiconductor film S2 serves as a main carrierpath. Since the density of defect states at the interface between theoxide semiconductor films S1 and S2 and the interface between the oxidesemiconductor films S2 and S3 can be made low, the influence ofinterface scattering on carrier conduction is small, and high on-statecurrent can be obtained.

When an electron is trapped in a trap state, the trapped electronbehaves like fixed charge; thus, the threshold voltage of the transistoris shifted in a positive direction. The oxide semiconductor films S1 andS3 can make the trap state apart from the oxide semiconductor film S2.This structure can prevent the positive shift of the threshold voltageof the transistor.

A material whose conductivity is sufficiently lower than that of theoxide semiconductor film S2 is used for the oxide semiconductor films S1and S3. Accordingly, the oxide semiconductor film S2, the interfacebetween the oxide semiconductor films S1 and S2, and the interfacebetween the oxide semiconductor films S2 and S3 mainly function as achannel region. For example, an oxide semiconductor film with highinsulation performance and the atomic ratio represented by the region Cin FIG. 21C can be used as the oxide semiconductor films S1 and S3. Notethat the region C illustrated in FIG. 21C represents atomic ratios[In]:[M]:[Zn] of 0:1:0, 1:3:2, and 1:3:4 and the vicinities thereof.

In the case where an oxide semiconductor film with the atomic ratiorepresented by the region A is used as the oxide semiconductor film S2,it is particularly preferable to use an oxide with an atomic ratio where[M]/[In] is greater than or equal to 1, preferably greater than or equalto 2 as each of the oxide semiconductor films S1 and S3. Moreover, theoxide semiconductor film S3 is preferably an oxide semiconductor filmhaving [M]/([Zn]+[In]) of 1 or greater to obtain sufficiently highinsulation performance.

Note that the structures described in this embodiment can be used incombination with any of the structures described in the otherembodiments as appropriate.

Embodiment 4

In this embodiment, a display module and electronic devices that includethe display device of one embodiment of the present invention will bedescribed with reference to FIG. 23, FIGS. 24A to 24E, and FIGS. 25A to25E.

<4-1. Display Module>

In a display module 8000 illustrated in FIG. 23, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a frame 8009, a printed circuit board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can beused for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may overlap with the display panel 8006. Alternatively,a counter substrate (sealing substrate) of the display panel 8006 canhave a touch panel function. Alternatively, a photosensor may beprovided in each pixel of the display panel 8006 so as to function as anoptical touch panel.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed circuit board 8010. The frame 8009 mayfunction as a radiator plate.

The printed circuit board 8010 is provided with a power supply circuitand a signal processing circuit for outputting a video signal and aclock signal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<4-2. Electronic Device>

FIGS. 24A to 24E and FIGS. 25A to 25E illustrate electronic devices.These electronic devices can include a housing 9000, a display portion9001, a camera 9002, a speaker 9003, an operation key 9005 (including apower switch or an operation switch), a connection terminal 9006, asensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 24A to 24E and FIGS. 25A to25E can have a variety of functions, for example, a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling a process with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions of the electronic devicesillustrated in FIGS. 24A to 24E and FIGS. 25A to 25E are not limitedthereto, and the electronic devices may have other functions.

The electronic devices illustrated in FIGS. 24A to 24E and FIGS. 25A to25E are described in detail below.

FIG. 24A is a perspective view illustrating a television device 9100.The television device 9100 can include the display portion 9001 having alarge screen size of, for example, 50 inches or more, 80 inches or more,or 100 inches or more.

FIG. 24B, FIG. 24C, FIG. 24D, and FIG. 24E are perspective viewsillustrating a portable information terminal 9101, a portableinformation terminal 9102, a portable information terminal 9103, and aportable information terminal 9104, respectively.

The portable information terminal 9101 illustrated in FIG. 24B has, forexample, one or more of a function of a telephone set, a notebook, andan information browsing system. Specifically, the portable informationterminal 9101 can be used as a smartphone. Although not illustrated, thespeaker 9003, the connection terminal 9006, the sensor 9007, and thelike may be provided in the portable information terminal 9101. Theportable information terminal 9101 can display characters and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons or simply icons) canbe displayed on one surface of the display portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed onanother surface (for example, a side surface) of the display portion9001. Examples of the information 9051 include notification from asocial networking service (SNS), display indicating reception of ane-mail or an incoming call, the title of the e-mail, the SNS, or thelike, the sender of the e-mail, the SNS, or the like, the date, thetime, remaining battery, and the strength of a received signal.Alternatively, the operation buttons 9050 or the like may be displayedin place of the information 9051. The display portion 9001 of theportable information terminal 9101 partly has a curved surface.

The portable information terminal 9102 illustrated in FIG. 24C has afunction of displaying information, for example, on three or more sidesof the display portion 9001. Here, information 9052, information 9053,and information 9054 are displayed on different sides. For example, auser of the portable information terminal 9102 can see the display(here, the information 9053) with the portable information terminal 9102put in a breast pocket of his/her clothes. Specifically, a caller'sphone number, name, or the like of an incoming call is displayed in aposition that can be seen from above the portable information terminal9102. Thus, the user can see the display without taking out the portableinformation terminal 9102 from the pocket and decide whether to answerthe call. The display portion 9001 of the portable information terminal9102 partly has a curved surface.

Unlike in the portable information terminals 9101 and 9102 describedabove, the display portion 9001 does not have a curved surface in theportable information terminal 9103 illustrated in FIG. 24D.

The display portion 9001 of the portable information terminal 9104illustrated in FIG. 24E is curved. As illustrated in FIG. 24E, it ispreferable that the portable information terminal 9104 be provided witha camera 9002 to have a function of taking a still image, a function oftaking a moving image, a function of storing the taken image in a memorymedium (an external memory medium or a memory medium incorporated in thecamera), a function of displaying the taken image on the display portion9001, or the like.

FIG. 25A is a perspective view of a watch-type portable informationterminal 9200. FIG. 25B is a perspective view of a watch-type portableinformation terminal 9201.

The portable information terminal 9200 illustrated in FIG. 25A iscapable of executing a variety of applications such as mobile phonecalls, e-mailing, viewing and editing texts, music reproduction,Internet communication, and computer games. The display surface of thedisplay portion 9001 is bent, and images can be displayed on the bentdisplay surface. The portable information terminal 9200 can employ nearfield communication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. Moreover, the portable information terminal 9200 includes theconnection terminal 9006, and data can be directly transmitted to andreceived from another information terminal via a connector. Chargingthrough the connection terminal 9006 is possible. Note that the chargingoperation may be performed by wireless power feeding without using theconnection terminal 9006.

Unlike in the portable information terminal 9200 illustrated in FIG.25A, the display surface of the display portion 9001 is not curved inthe portable information terminal 9201 illustrated in FIG. 25B.Furthermore, the external state of the display portion of the portableinformation terminal 9201 is a non-rectangular shape (a circular shapein FIG. 25B).

FIGS. 25C, 25D, and 25E are perspective views of a foldable portableinformation terminal 9202. FIG. 25C is a perspective view illustratingthe portable information terminal 9202 that is opened. FIG. 25D is aperspective view illustrating the portable information terminal 9202that is being opened or being folded. FIG. 25E is a perspective viewillustrating the portable information terminal 9202 that is folded.

The folded portable information terminal 9202 is highly portable, andthe opened portable information terminal 9202 is highly browsable due toa seamless large display region. The display portion 9001 of theportable information terminal 9202 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9202 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9202 can be reversiblychanged in shape from opened to folded. For example, the portableinformation terminal 9202 can be bent with a radius of curvature ofgreater than or equal to 1 mm and less than or equal to 150 mm.

The display device which is one embodiment of the present invention canbe preferably used for the display portion 9001.

Electronic devices described in this embodiment are characterized byhaving a display portion for displaying some sort of information. Notethat the semiconductor device of one embodiment of the present inventioncan also be used for an electronic appliance that does not have adisplay portion.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Example

A display device including the pixels 10 with the structure illustratedin FIGS. 1A and 1B in Embodiment 1 was fabricated, and the displayresults of the display device were evaluated.

Tables 1 and 2 show specifications of the display device fabricated inthis example.

TABLE 1 Specifications Panel size 4.3 inches Number of effective pixels1080 (H) × 1920 (V) Pixel size 49.5 μm (H) × 49.5 μm (V) External panelsize 76.0 mm (H) × 130.0 mm (V) Display region 53.46 mm (H) × 95.04 mm(V) Resolution 513 ppi LCD Reflective twisted ECB mode Aperture ratio56.7% Drive frequency 60 Hz Video signal format Analog line sequentialGate driver Built-in Source driver Driver IC

TABLE 2 Specifications Panel size 4.3 inches Number of effective pixels1080 (H) × 1920 (V) Pixel size 49.5 μm (H) × 49.5 μm (V) External panelsize 76.0 mm (H) × 130.0 mm (V) Display region 53.46 mm (H) × 95.04 mm(V) Resolution 513 ppi OLED Bottom emission (Tandem white + CF) Apertureratio R: 25.6%, G: 25.2%, B: 25.1%, and W: 5.7%, Average: 20.4% Drivefrequency 60 Hz Video signal format Analog line sequential Gate driverBuilt-in Source driver Driver IC

Table 1 shows the specifications of the display device including the LCDelements. Table 2 shows the specifications of the display deviceincluding the OLED elements.

<Results of Display on Display Device>

Next, results of display on the display device fabricated in thisexample is shown in FIGS. 26A and 26B. FIG. 26A shows display results ofa display mode (color display) using an OLED element that is alight-emitting element. FIG. 26B shows display results of a display mode(monochrome display) using a reflective LCD element.

As shown in FIGS. 26A and 26B, it was confirmed that the display deviceof one embodiment of the present invention had a favorable displayquality.

Note that the structures described in this example can be used incombination with any of the structures described in the otherembodiments as appropriate.

This application is based on Japanese Patent Application serial no.2016-089154 filed with Japan Patent Office on Apr. 27, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first displayelement; a second display element; a first transistor; a secondtransistor; and a third transistor, wherein the first display elementcomprises a liquid crystal layer, wherein the second display elementcomprises a light-emitting layer, wherein the first transistor isconfigured to select the first display element, wherein the secondtransistor is configured to select the second display element, whereinthe third transistor is configured to control driving of the seconddisplay element, wherein the first transistor and the second transistorare located on a same surface, and wherein the third transistor islocated over the first transistor and the second transistor andcomprises one of a source electrode and a drain electrode of the secondtransistor as a gate electrode.
 2. The display device according to claim1, wherein the third transistor comprises a plurality of gateelectrodes.
 3. The display device according to claim 1, wherein one ormore of the first transistor, the second transistor, and the thirdtransistor comprise an oxide semiconductor in a semiconductor layer. 4.The display device according to claim 1, wherein the light-emittinglayer is configured to emit light toward the liquid crystal layer side.5. A display module comprising: the display device according to theclaim 1; and a touch sensor.
 6. An electronic device comprising: thedisplay module according to claim 5; and an operation key or a battery.7. An electronic device comprising: the display device according toclaim 1; and an operation key or a battery.
 8. A display devicecomprising: a first display element; a second display element; a firsttransistor; a second transistor; a third transistor; and a capacitor,wherein the first display element comprises a first pixel electrode anda liquid crystal layer, wherein the second display element comprises asecond pixel electrode and a light-emitting layer, wherein the firsttransistor is electrically connected to the first pixel electrode,wherein the second transistor is electrically connected to the secondpixel electrode, wherein the third transistor is electrically connectedto the second display element, wherein the capacitor comprises a pair ofelectrodes, wherein one of the pair of electrodes comprises a capacitorelectrode, wherein the other of the pair of electrodes comprises thefirst pixel electrode, wherein the first transistor and the secondtransistor are located on a same surface, and wherein the thirdtransistor is located over the first transistor and the secondtransistor and comprises one of a source electrode and a drain electrodeof the second transistor as a gate electrode.
 9. The display deviceaccording to claim 8, wherein the capacitor electrode is positionedbelow one or both of the first transistor and the second transistor. 10.The display device according to claim 8, wherein the first pixelelectrode is configured to reflect light, and wherein the second pixelelectrode is configured to transmit light.
 11. The display deviceaccording to claim 8, wherein the first pixel electrode comprises one orboth of silver and aluminum, and wherein the second pixel electrodecomprises one or more of indium, zinc, tin, and silicon.
 12. The displaydevice according to claim 8, wherein the third transistor comprises aplurality of gate electrodes.
 13. The display device according to claim8, wherein one or more of the first transistor, the second transistor,and the third transistor comprise an oxide semiconductor in asemiconductor layer.
 14. The display device according to claim 8,wherein the light-emitting layer is configured to emit light toward theliquid crystal layer side.
 15. A display module comprising: the displaydevice according to claim 8; and a touch sensor.
 16. An electronicdevice comprising: the display module according to claim 15; and anoperation key or a battery.
 17. An electronic device comprising: thedisplay device according to claim 8; and an operation key or a battery.